feat(auth): add anthropic api key storage and cli auth

src/auth/anthropic.ts

@@ -0,0 +1,72 @@
+import { readFileSync, writeFileSync, mkdirSync, chmodSync } from 'fs';
+import { resolve } from 'path';
+import { homedir } from 'os';
+
+const AUTH_DIR = resolve(homedir(), '.config/flynn');
+const AUTH_FILE = resolve(AUTH_DIR, 'auth.json');
+
+export interface AnthropicAuthInfo {
+  /** Anthropic API key. */
+  api_key: string;
+  created_at: string;
+}
+
+interface AuthStore {
+  anthropic?: AnthropicAuthInfo;
+  [key: string]: unknown;
+}
+
+function safeJsonParse<T>(raw: string): T | null {
+  try {
+    return JSON.parse(raw) as T;
+  } catch {
+    return null;
+  }
+}
+
+function readAuthStore(): AuthStore {
+  try {
+    const raw = readFileSync(AUTH_FILE, 'utf-8');
+    const parsed = safeJsonParse<AuthStore>(raw);
+    return parsed ?? {};
+  } catch {
+    return {};
+  }
+}
+
+function writeAuthStore(store: AuthStore): void {
+  mkdirSync(AUTH_DIR, { recursive: true });
+  writeFileSync(AUTH_FILE, JSON.stringify(store, null, 2) + '\n', 'utf-8');
+  chmodSync(AUTH_FILE, 0o600);
+}
+
+export function loadStoredAnthropicAuth(): AnthropicAuthInfo | null {
+  const store = readAuthStore();
+  return store.anthropic ?? null;
+}
+
+export function storeAnthropicAuth(apiKey: string): void {
+  const trimmed = apiKey.trim();
+  if (!trimmed) {
+    throw new Error('Anthropic API key is empty');
+  }
+  const store = readAuthStore();
+  store.anthropic = { api_key: trimmed, created_at: new Date().toISOString() };
+  writeAuthStore(store);
+}
+
+export function clearAnthropicAuth(): void {
+  const store = readAuthStore();
+  delete store.anthropic;
+  writeAuthStore(store);
+}
+
+/**
+ * Get an Anthropic API key from any available source.
+ * Priority: ANTHROPIC_API_KEY → stored auth.json.
+ */
+export function getAnthropicApiKey(): string | null {
+  return process.env.ANTHROPIC_API_KEY
+    ?? loadStoredAnthropicAuth()?.api_key
+    ?? null;
+}